Damage simulation in composite materials:

why it matters and what is happening currently at NASA in this area

Mack McElroy1, Nelson de Carvalho2, Ashley Estes3, Shih-yung Lin4

1Mack McElroy currently works at Johnson Space Center in the Orion program focusing on fracture control and composite

materials. Previously Mack worked at NASA Langley researching damage simulation in composite laminates. While at

Langley, Mack completed a PhD in Aerospace Engineering from North Carolina State University. His work experience also

includes structural design and analysis in the ship building and gas turbine industries. 2NASA Langley Research Center (National Institute of Aerospace) 3Johnson Space Center (Jacobs) 4NASA Langley Research Center

Use of lightweight composite materials in space and aircraft structure designs is often challenging due to high costs

associated with structural certification. Of primary concern in the use of composite structures is durability and damage

tolerance. This concern is due to the inherent susceptibility of composite materials to both fabrication and service

induced flaws. Due to a lack of general industry accepted analysis tools applicable to composites damage simulation, a

certification procedure relies almost entirely on testing. It is this reliance on testing, especially compared to structures

comprised of legacy metallic materials where damage simulation tools are available, that can drive costs for using

composite materials in aerospace structures.

The observation that use of composites can be expensive due to testing requirements is not new and as such, research

on analysis tools for simulating damage in composite structures has been occurring for several decades. A convenient

approach many researchers/model-developers in this area have taken is to select a specific problem relevant to

aerospace structural certification and develop a model that is accurate within that scope. Some examples are open hole

tension tests, compression after impact tests, low-velocity impact, damage tolerance of an embedded flaw, and fatigue

crack growth to name a few. Based on the premise that running analyses is cheaper than running tests, one motivation

that many researchers in this area have is that if generally applicable and reliable damage simulation tools were

available the dependence on certification testing could be lessened thereby reducing overall design cost. It is generally

accepted that simulation tools if applied in this manner would still need to be thoroughly validated and that composite

testing will never be completely replaced by analysis.

Research and development is currently occurring at NASA to create numerical damage simulation tools applicable to

damage in composites. The Advanced Composites Project (ACP) at NASA Langley has supported the development of

composites damage simulation tools in a consortium of aerospace companies with a goal of reducing the certification

time of a commercial aircraft by 30%. And while the scope of ACP does not include spacecraft, much of the methodology

and simulation capabilities can apply to spacecraft certification in the Space Launch System and Orion programs as well.

Some specific applications of composite damage simulation models in a certification program are (1) evaluation of

damage during service when maintenance may be difficult or impossible, (2) a tool for early design iterations, (3) gaining

insight into a particular damage process and applying this insight towards a test coupon or structural design, and (4)

analysis of damage scenarios that are difficult or impossible to recreate in a test. As analysis capabilities improve, these

applications and more will become realized resulting in a reduction in cost for use of composites in aerospace vehicles.

NASA is engaged in this process from both research and application perspectives. In addition to the background

information discussed previously, this presentation covers a look at recent research at NASA in this area and some

current/potential applications in the Orion program.

https://ntrs.nasa.gov/search.jsp?R=20170007397 2018-06-26T16:43:36+00:00Z

Damage simulation in composite

materials: Why it matters and

what is happening currently at

NASA in this area

Mack McElroy

Nelson de Carvalho

Ashley Estes

Shih-yung Lin

1

August 2017

Background

2

Composites are susceptible to manufacturing flaws and damage from transverse loads

Damage may not be visible externally but still cause a reduction in strength

delamination

matrix crack

delaminationstransverse matrix cracks

impacted surface

2 mm

Example 2

Example 1

Ultrasonic scan of impact damage (delamination at multiple ply interfaces)

Example 3

Background

3

Design and certification process for composite

aerospace structures:

Heavily reliant on tests

Damage simulation tools may reduce the need for some testing manufacturing flaw compression after impact worst case credible damage damage initiation

Expensive & time consuming

Preliminary

Design

Detail

DesignCertification

Testing

Simulation - existingSimulation – desired

Testing

damage tolerance

What has NASA done in the past?

4

Example 21970 1980 1990 2000 2010 2017

Advanced Composites Technology

DC-XA & X-33

High Speed Research

CCM1

Aircraft Energy Efficiency

ACP2

Composite material advances

Non-destructive evaluation Fabrication technology

Numerical simulation

Areas of research

Source: Tenney, D.R., Davis, J.G., Pipes, R.B, Johnston, N. 2009. NASA composite materials development: lessons learned and future challenges. NASA Report LF99-9370.

1Composite Crew Module2Advanced Composites Project

5

1970s 1980s 1990s

Work to improve material toughness

Hand layup fabrication

First composite aircraft structures:

Damage tolerant designs

Toughened materials

Advanced tape placement machines

Composite interlaminarfracture tests:

Automatic fiber placement machines

Textile evaluations

Structural analysis and design methods

Stitched composites

Cost efficient primary structures:

DCB

ENF

Edge delam. tensile (LaRC) Cracked

lap shear

2000 - present

Primary aircraft structures:

Advanced fabrication capabilities:

Advanced numerical simulations:

All Nippon Airways Boeing 787-8 (JA801A) at Okayama Airport. October 2011.

[All Nippon…]

[Harris]

[Harris]

[Tenney]

[Tenney]

What has NASA done in the past?

What is NASA doing now?

6

Example 2

Tool development (selected)

(1) Adaptive Fidelity Shell, 2014-present (M. McElroy) Advanced Composites Project Space Act Agreement: Swerea SICOMP Space Act Agreement: Rice University Space Act Agreement: North Carolina State University

(2) Extended interface element, 2013-present (N. de Carvalho) Advanced Composites Project Advanced Composites Consortium

Application

(1) Orion back shell (A. Estes)

(2) Orion heatshield (NESC)

Advanced Composites Project (LaRC, 2015-2019)

Adaptive Fidelity Shell Model

7

ΩA

(1) Undamaged ElementExample

2

ΩA

ΩB

(2) Split Element

*Chen, B.Y., Pinho, S.T., De Carvalho N.V., Baiz, P.M., Tay, T.E. 2014. “A Floating Node Method for the

Modelling of Discontinuities in Composites,” Engineering Fracture Mechanics 127:104-134.

= RN and unused FN

= floating node (FN)

= real node (RN)

Element formulation summary: Floating Node Method* + VCCT

Model developer: Mack McElroy (JSC)

Key features:

• Discrete, mesh-independent, representation of delaminations and transverse matrix cracks

• Low(er) mesh fidelity• High computational efficiency• User friendly

Cost effective analysis tool

Adaptive Fidelity Shell Model

8

Mesh size Runtime1.0 mm 37 minutes2.5 mm 6 minutes5.0 mm 1.5 minutes

1.0 mm 31 hours

AFS

High fidelity [Krueger]

Example 1: Double cantilever beam

migration

Prescribed

displacement

Example 2: Delamination Migration

Test: initial delamination

Test: delaminations after impact

AF Shell simulation

impact test dataAFS model

Example 3: Low-velocity impact (progressive damage)

. . .

Extended Interface Element

9

Interface

Sub-element 1

Sub-element 2

Floating node

Real node

One extended interface element Illustration of matrix crack/interface kinematics

Key features:

• Discrete, mesh-independent, representation of crack tip kinematics (matrix cracks/delaminations/interaction)

• Discrete crack approach compatible with both CZ/VCCT (quasi-static/fatigue)• Unlimited number of cracks (crack density not set ‘a priori’)

Model developer: Nelson de Carvalho (LaRC, NIA)

Extended Interface Element

10

migrationattemptsmatrixcrackleadingtomigration

Initialmatrixcrack

Example 1: Delamination/matrix crack interaction

0

100

200

300

400

500

600

700

0 0.5 1

Force

[N]

U [mm]

Expt-1

YT=127

YT=64

Experiments

YT = 127 MPa

YT = 64 MPa

Peak

Load

Test setupSimulation results

Example 2: “Skin-stringer debonding”

Detail

skin

stringer flange

11

Applications: Composites on Orion

Crew Module

Crew Module Adapter

European Service Module

Service Module Adapter

Launch Abort System (LAS)

Serv

ice

Mo

du

le

Space Launch System

ECLSS Wall(internal)

SMA

Backshell

Photo: LM

heat-shield

Photo: LM

CMA

LAS Fairing

Photo: LM

LAS Ogive

Photo: LM

Orion Backshell

12

1

2

3

94

5

6

7

8

1716

15

14

13

12

11

10

Orion crew module

Panel F: composite sandwich

Finite element model where damage tolerance of embedded flaws can be evaluated (VCCT)

Difficult to test Flight loads can be applied Any flaw size and location can be evaluated Quick evaluation of design changes

Analyst: Ashley Estes (JSC, Jacobs)

Panel F Finite element model (flaw locations identified)

Flaw mesh detail

D = 0.25”

D = 0.50”D = 1.00”

Orion Heatshield

13

Analyst: NESC

Thermal tiles (AVCOAT) bonded to heatshield carrier structure

Damage tolerance concerns in AVCOAT tiles and at bondline

Material characterization Model validation Full scale model with embedded flaws in

heatshield Re-entry thermal/mechanical loads applied Equivalent test is not possible

flaws

Summary

14

Certification of composite aerospace vehicles is time consuming and expensive

Composite damage simulation tools may lower certification expenses by reducing the amount of testing Tool development Application & integration

into design/analysispractices

Two main challenges to realize benefits

Summary: What is NASA doing?

15

Advanced composites project (LaRC)

Tool development (selected) Extended interface element (de Carvalho, LaRC) Adaptive fidelity shell element (McElroy, JSC)

Application Orion backshell damage tolerance Orion heatshield damage tolerance

Summary: What isn’t NASA doing?

16

Effective agency wide sharing of state-of-the-art software tools

Development of engineering tools for composites damage simulation/fracture control

Material characterization of non-metallic materials for model validation

Integration of composites damage simulation into standard fracture control and M&P practices (Orion)

Questions?

17

Mack McElroy (JSC)mark.w.mcelroy@nasa.gov

Other contributors:Nelson de Carvalho (LaRC, NIA)

Ashley Estes (JSC, Jacobs)Shih-yung Lin (LaRC)